The present invention relates to a multiport memory.
A multiport memory is generally sued for image processing. Multiport memories used with high performance work stations, typically engineering work stations (EWS) or graphic work stations (GWS) are required to be accessible by CPU for 100% of the period while data are outputted to a display at high speed in order to realize high speed image processing and high speed image overwrite. Such a multiport memory is generally constructed with a random access memory port (hereinafter abbreviated as RAM port) and a serial access memory port (hereinafter abbreviated as SAM port). A RAM port is constructed with a memory cell array and its peripheral circuit, and a SAM port is constructed with a data register, serial data selector and their peripheral circuits. A multiport memory has a circuit arrangement that allows data transfer between a memory cell of the RAM port and the data register of the SAM port.
Such data transfer will be described with reference to FIG. 2. As the data stored in cell (not shown) of a RAM port is read, the potentials at bit lines BL10 and BL10 connected to the memory cell changes. This change is potential is detected by a sense amplifier which is constructed with N-channel MOS transistors TN21, TN22, and TN23. Thereafter, a restore circuit 10 drives one of the bit lines BL10 and BL10 to a potential "H" and the other to a potential "L", the restore circuit 10 being constructed of P-channel MOS transistors TP11, and TP12. In the data transfer mode, the potential (data in the memory cell) at the bit lines BL10 and BL10 are sent via transfer gates TRG1 and TRG2 to a data register 30 which is constructed of P-channel MOS transistors TP31 and TP32 and N-channel MOS transistors TN31 and TN32. The data are then sent via bit lines BL12 and BL12 and a serial data selector (not shown) to an external display. SAP in FIG. 2 indicates a restore control signal for bit lines, and .phi..sub.1 indicates a sense amplifier control signal.
It is known in the art that in a general purpose DRAM, barrier transistors T1 and T2 are provided between the restore circuit 10 and sense amplifier 20 to sense data at high speed (refer to Japanese Laid-open No. 62-165787/1987). The sense operation by a RAM port of a conventional multiport memory is quite the same as that of a general purpose DRAM. Therefore, as shown in FIG. 3, it is possible to allow high speed operation at bit lines by providing barrier transistors T1 and T2 between a restore circuit 10 and sense amplifier 20.
With the barrier transistors T1 and T2 connected between the restore circuit 10 and sense amplifier 20, the restore circuit 10 drives one of the bit lines BL10 and BL10, e.g., bit line BL10, directly connected to the memory cell to a "H" state of Vcc potential. However, the potential at a node BL11 on the a sense amplifier 20 side rises only to a value (Vcc-V.sub.T) because of the threshold value V.sub.T of the barrier transistor T1. Consider the case where an inverse data is transferred from the RAM port to the SAM port, e.g., the case where the data with the potential "H" at the bit line BL10 and "L" at the bit line BL10 is transferred under the condition that the potential at a node BL12 of the SAM port is "L" and that at a node BL12 is "H". In such a case there may occur an erroneous data transfer may occur. For example, assuming that the potential at the node BL11 is "L" and that at the node BL12 is "H", if the transfer gate TRG2 is turned or such that the potential at the node BL12 is made "L" by the on-state transistor TN22 of the sense amplifier 20, charges are transferred to the nodes BL11 and BL12 via the transistor TP32 of the data register 30. Since the barrier transistors T1 and T2 are provided for realizing high speed sensing, the capacitance of the C.sub.BL11 of the node BL11 is smaller than the capacitance C.sub.BL12 of the node BL12. Accordingly, as charges are transferred from the transistor TP32 to the nodes BL11 and BL12, the potential at the node BL11 rises by a ratio of capacitances of the nodes BL11 and BL12, and the off-state transistor TN21 turns on. The potential at the node BL11 which has been in an "H" state becomes "L" so that the potential at node BL12 cannot be made "H". In other words, the potential on the SAM port side cannot be inverted and hence data transfer is impossible.